I think what he is describing here is not the actuall courage of Mrs Dubose battling her sickness rather how she managed to stick to her beliefs and views even when Jem tried to contradict her. Atticus does not go on and disregard her views even though they oppose eachother and instead choses to respect them. Teaching Jem a valuble lesson on how to treat your enemies by showing this kind of respect towards eachother.
Mrs. Dubose is facing her addiction head on with no knowledge of what the outcome may be. She is gooing to fight her battle blindly, no matter how scary or difficult it becomes. A man with a "gun in his hand" enters his battle with an advantage, and no fear of the outcome. A gun would make a fight less courageous due to the obvious advantage, and lack of courage needed to engage in the fight.